Input cap (2.2uF) / 50k pot (bass roll-off) question: - diyAudio
 Input cap (2.2uF) / 50k pot (bass roll-off) question:
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 23rd September 2003, 05:25 PM #2 diyAudio Member   Join Date: Jun 2003 Location: Near Baltimore, MD Anyone? __________________ I had the right to remain silent. I just didn't have the ability. (Comedian Ron White)
 23rd September 2003, 05:52 PM #3 diyAudio Member     Join Date: Apr 2003 Location: Cambridge you can work out the -3dB pont quite easily for this, if you multiply the value of the capacitor by the value of the input resistance and then that value by two times pi. Then take that value and divide one by it you will get your -3db frequency ( sorry that is about as clear as a brick, if you want ask and I will go through it again). So, assuming you don't have a low value resistor to ground after your volume pot, the -3dB point of your system would be at about 1.4hZ which should be ok for most sets of speakers.
 23rd September 2003, 06:15 PM #4 diyAudio Moderator Emeritus     Join Date: Sep 2002 Location: Germany -3dB point ( in Hz ) = 1/(2 xpi x C x R ) 1/( 2 x pi x 2.2exp10-6 x 50000 ) = 1.448 Herz This assuming the input resistance of the chip is infinite which it isn't. Check the datasheet for that and parallel that resistance to that of the potentiometer. Suppose the chips input impedance is 50 k as well ( just for the calculation ). 1/ ( 2 x pi x 2.2exp10-6 x 25000 ) = 2.89 Herz. So it's pretty obvious the cap's value is not the problem. Maybe oscillation is . When not it can be the subjective impression of the cap/potentiometer combination. Search for oscillation first as it will very likely be the problem. Please consider the tips Joe Rasmussen gave for wiring and/or the new revised schematic on the Amp chip forum. __________________ It's only audio. Member of the non modular PCB design committee
 23rd September 2003, 06:36 PM #5 diyAudio Member   Join Date: Jun 2003 Location: Near Baltimore, MD Thanks to both of you for the pointers... but how does one tell if a chip is oscillating? __________________ I had the right to remain silent. I just didn't have the ability. (Comedian Ron White)
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Join Date: Jun 2002
If used, as GnD said, with IGC (Inverted GainClone), and with pot at the minimum position (or at maximum position assuming zero source impedance), the impedance that 2.2uF input cap sees is mainly Rin, which is usually 10k. Hence, at 20Hz roll-off will be about 0.5dB and this is followed by about 20° phase shift. The -3dB (and 45° phase shift) point will be at 7.2Hz. In all other cases the whole slope goes lower.

So, I’d say 2.2uF is more or less acceptable as an input cap for IGC.

Quote:
 Originally posted by GrahamnDodder Could that cap/pot combo be my problem?
No.

Quote:
 Or should I move on and look for other culprits…
Yes.

Quote:
 …which could, of course, be the speakers.
This hardly has anything with speakers.

Pedja

diyAudio Moderator Emeritus

Join Date: Sep 2002
Location: Germany
Quote:
 If used, as GnD said, with IGC (Inverted GainClone), and with pot at the minimum position (or at maximum position assuming zero source impedance), the impedance that 2.2uF input cap sees is mainly Rin, which is usually 10k. Hence, at 20Hz roll-off will be about 0.5dB and this is followed by about 20° phase shift. The -3dB (and 45° phase shift) point will be at 7.2Hz. In all other cases the whole slope goes lower.
Right, my Gainclone to be is non inverting so I made a wrong assumption. GnD clearly wrote IGC. The outcome is nevertheless practically the same, it is not the cap value that causes the problem. But it won't hurt to use a 4.7 uF cap if you want to try it. Do check for oscillation with an oscilloscope . You won't be the first with an oscillating (I)GC.

No signal in and still signal ( mostly higher frequencies or even RF ) at the output means it oscillates.
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